Near-IR light is ideally suited to the optical imaging of live cells and whole animals because of the lower autofluorescence, lower phototoxicity, and greater tissue penetration in this spectral region. However, there are no near-IR fluorescent proteins, and exogenous near-IR fluorophores either lack cell permeability or give rise to extensive background labeling within cells. This has greatly limited the ability to use near-IR light to image intracellular events. For example, exemplary sulfonated near-IR fluorophores such as Cy5 are not cell permeable and have been largely restricted to extracellular applications. Furthermore, near-IR fluorophores have been recalcitrant to the methods generally used to trigger the activation of fluorescence. This has limited the ability of near-IR fluorophores to report on enzymatic activity or be responsive to photoactivation. In this grant, we plan a rational and straightforward approach to 1) deliver exemplary sulfonated near-IR fluorophores into cells, 2) create photoactivatable near-IR fluorophores that can be used to study the dynamic behavior of biomolecules and cells, and 3) detect enzymatic activity in live cells using near-IR light. Together, the near-IR fluorescent tools we develop will bring the significant advantages of near-IR imaging to the study of dynamic and enzymatic processes in live cells, with major potential applications for both basic research and medical imaging. PUBLIC HEALTH RELEVANCE: Live imaging of cells with light is a simple and rapid way to observe the inner workings of cell function. This study will improve the ability of light-based imaging to be used for the study and diagnosis of disease.